Zirconium (ZrO2) is well-known structural ceramic, which exhibits tetragonal-to-monoclinic martensitic phase transformation. This phase transformation is of technological importance as it contributes to the toughening of ceramics. Porter, D. L., Evans, A. G., Heuer, A. H., Acta Metall, 1979, 27, 1649. The tetragonal-to-monoclinic phase transformation is accompanied by volume expansion and can be triggered by hydrostatic and shear stresses. See Simha, N., Truskinovsky, L. Acta Metall. Mater. 1994, 42, 3827.
This type of transformation can take place near the crack tip due to local stress concentration, which can blunt the crack tip as a result of the development of compressive stresses associated with the phase transformation; thus, delaying the crack propagation and resisting subsequent fracture.
Zirconia is used as a dispersed phase in oxide (alumina) (Kosmac, T, Swain, M. V. Claussen, N, Mater. Sci Eng. 1984, 71, 57), as well as non-oxide (carbides, borides, and nitrides) (Swain, M. V. Mater. Forum 1988, 11, 182.) ceramics to increase their fracture toughness, strength, and hardness. This stress induced transformation has also been shown to increase the plasticity from the shape deformation and accommodation strains, which are associated with the transformation. (Muddle, B. C. Kelly, P. M. MaterForum 1988, 11, 182.) Increase in the wear and corrosion resistance of nanostructured ceramic coatings as a result of stress-induced phase transformation has also been reported. (Aita, C. R., In Advances in Catings: TMS Annual Meeting, Warrendale, Pa., 1995 p. 235) Besides this, tetragonal ZrO2 also finds application as catalyst/catalyst support for various gas-phase reactions. (Haruta, M. Kobayashi, T, Sano, H., Yamada, N Chem Lett. 1987, 829, 405 and Knell, A, Barnickel, P, Baiker, A, Wokaum, A. J. Catal. 1992, 137, 306.) Hence, synthesizing ZrO2 particles with metastable tetragonal crystal structure is important.
Doping zirconia with trivalent impurities has been a traditional approach for the metastable tetragonal phase stabilization. (Ping, L, Chen, L. W., Penner-Hahn, J. E., J. Am Ceram Soc. 1994, 77, 188.) Particle size has also been observed to have its own effect on the metastable tetragonal phase stability in nanocrystalline zirconia. (Garvie, R. C., J. Phys. Chem 1965, 69, 1238, and Nitsche, R, Rodewald, M, Skandan, G, Guess, H., Hahn, H. Nanostruct. Mater. 1996, 7, 535, and Nitsche, R, Winterer, M, Hahn, H, Nanostruct. Mater. 1995, 6, 1979). Due to the existence of a critical nanoparticle size for the metastable tetragonal phase stabilization, the tetragonal phase stabilization within the submicron sized ZrO2 particles has been unachievable.